The oil and gas industry is continuously searching for more dispersed, deeper and smaller fields under the sea and further offshore. This has led to a strong interest in the development of subsea technology that can enable cost-effective deep sea oil/gas exploration and recovery. Today, power electronics for driving large compressor and pump loads are located at atmospheric pressures either on-shore or on a platform with long cable connections. In the past few years, a pressure compensated concept has been extensively discussed for subsea technology. In a pressure compensated system, the electrical components (e.g. power conversion unit) and associated electronic circuits will be put under pressure close to or equal to the external pressure (which is high, e.g. 300 bar at 3000 meters depth) in a pressure compensated vessel filled with a dielectric liquid medium. The same type of pressure compensated systems is also used for subsea installations of static electric induction devices such as subsea transformers.
The development of such a pressure compensated system generates unique challenges with respect to the design of the electrical components and cooling of the entire system. One of the critical components of such a system is the liquid medium itself. This is because the dielectric breakdown behavior and thermal properties of the liquid at high pressures and low temperatures will determine the reliability of the power conversion unit and the size of the pressure compensated enclosure. Literature mentions that the dielectric properties of an insulating liquid are generally enhanced under high pressure conditions, see for example M. Koch et al., “The breakdown voltage of insulation oil under the influence of humidity, acidity, particles and pressure”, International Conference of APTADM, 2007, or R. Badent et al., “The effect of hydrostatic pressure on streamer inception and propagation in insulating oil”, Conference Record of the IEEE International Symposium on Electrical Insulation, 1994. However, the thermal properties of an insulating liquid at high pressures have to be understood and addressed appropriately during the design of the pressure compensated system.
It is well known that the viscosity of a liquid increases at low temperatures. This is shown in the graph of FIG. 1 illustrating variation of kinematic viscosity of three dielectric oils (natural ester, synthetic ester, and mineral transformer oil) with respect to temperature. In addition, the viscosity is also reported to increase significantly under high pressures, see for example D. L. Hogenboom et al., “Viscosity of several liquid hydrocarbons as a function of temperature, pressure and free volume”, The Journal of Chemical Physics, Vol. 46, No. 7, pp. 2586-2598, 1967. As a result, the cooling performance of the liquid at high pressures and low temperatures is a cause for concern.
Often, to achieve high reliability, cooling solutions based on natural convection are sought. In such a case, high viscosity of the cooling medium lowers the flow rates and thus the cooling performance. If, on the other hand, pumped cooling is chosen, the high viscosity of the cooling medium increases the needed pumping power. Another critical material property of the liquid is the thermal conductivity. For a sea water cooled natural convection cooling system intended to work under high pressure, the low oil heat transfer coefficient typically represents the highest thermal resistance in the system. A high thermal conductivity is therefore a significant advantage for the cooling medium.
EP 2717401 A1 relates to a subsea electrical power system comprising a first and a second subsea electrical device, and to a method of providing a subsea electrical power system.
US 2002/139962 A1 relates to dielectric fluids for use in transformers. In particular, it relates to dielectric isoparaffinic based transformer fluids.
Petro-Canada: “Luminol Product from Petro-Canada for Subsea Oil Gas Application” very briefly describes “Luminol”, or a luminol synthesized isoparaffin fluid. This document refers to the subsea oil and gas industry but does not explicitly mention a pressure compensated subsea arrangement.
Hence, there is still a need for an efficient dielectric liquid to be used in subsea power systems.